I love the Schramsberg Winery tour. That one in my opinion is very special because you get to go through the wine caves which are just lined with aging Champagne bottles and barrels. It's really beautiful and natural (some of the other tours that go through wine caves the caves are really modern and sterile which I don't like). They talk about the unique process of making Champagne and how they hand riddle all the bottles which most big sparkling wine companies don't do any more. Then there's a really lovely seated tasting where you can taste different cuvees and vintages.

I also really like the set-up at Benziger Winery in Glen Ellen on the Sonoma side, which is more of a self-guided tour. I think it's nice to balance guided tours and stuff you can do on your own at your own pace when doing a trip through Napa/Sonoma. The Benziger grounds have displays that show vines at various ages so you can see how they start out and develop, and how they are pruned/trellissed. They also have an insectarium and information on organic and/or biodynamic farming practices (they cultivate different insects to control pests rather than use damaging chemical pesticides). Very beautiful grounds also with picnic tables where you can have lunch.

One of my other favorites is Chateau Montelena, which is near Schramsberg. I don't know that they do their scheduled tour anymore, but if you can find a way to get a tour there, it's a beautiful property with a really interesting history. Their estate vineyard is part of the property also, with these amazing 30+year Cabernet vines sitting right at the foot of Mt. St. Helena, and it's wonderful to hear about their unique soils and climate and how they farm them, etc.

For anyone that's considering taking a trip out to Napa or another wine country region, I recommend looking at this article for some tips:
http://www.easyfoodandwine.com/Visiting-Wine-Country.html